The researchers used a two-factor, three-level completely randomized block design. IAA was applied at 0, 150, and 300 mg/L, and 6-BA at 0, 100, and 200 mg/L, producing every combination of the two hormones across the treated seedlings. After treatment, the team measured the contents of four endogenous hormones—IAA, abscisic acid (ABA), gibberellin (GA₃), and zeatin (ZA)—and analyzed how these hormones, and the ratios between them, correlated with sprout number and sprout length.

The results point to a clear dose-dependent pattern for auxin alone. When IAA was applied on its own, it exerted an overall inhibitory effect on sprout number that grew stronger with concentration. In parallel, the seedlings’ endogenous IAA content fell continuously as the exogenous IAA concentration rose. Two other hormones, GA₃ and ZA, responded non-monotonically: low concentrations promoted their levels while high concentrations suppressed them, a pattern the authors describe as “promotion at low concentration and inhibition at high concentration.”

Single applications of the cytokinin 6-BA told a different story. Under 6-BA treatment alone, the responses of sprout number and sprout length were inconsistent with each other, suggesting that this hormone does not act uniformly on the two sprout traits. Endogenous ZA content, however, declined continuously as the applied 6-BA concentration increased, indicating a steady downward shift in this cytokinin pool regardless of the mixed effects on growth.

The most striking findings emerged from the combined treatments. Applying IAA and 6-BA together produced a significant non-additive interaction effect, meaning the outcome of the mixture could not be predicted by simply adding up the effects of each hormone applied separately.

To move beyond simple correlations, the researchers used path analysis to quantify how strongly each hormone variable related to the sprout traits. Sprout number showed its strongest correlation with the ratio of GA₃ to ZA, while sprout length was significantly positively correlated with the summed content of ZA, GA₃, and IAA. Overall, the relative ratios among hormones explained sprout phenotypes better than the absolute content of any single hormone.

The authors conclude that germination and growth of sprouts in stumped P. yunnanensis are closely tied to the relative balance of endogenous hormones, and that combined application of exogenous IAA and 6-BA may influence sprout occurrence by regulating key ratios such as GA₃/ZA. Because the study measured hormone contents and sprout outcomes under controlled spray treatments, the findings describe associations within this experimental system rather than established mechanisms of action.

The practical significance lies in clonal propagation. Yunnan pine is propagated asexually through cuttings, and producing vigorous scions depends on generating sufficient sprouts from stumped stock plants. By identifying which hormone ratios track sprout number and length, the study offers quantitative parameters that could guide hormone application schemes aimed at efficient propagation of superior clones. The work remains an early-access, peer-reviewed accepted article, subject to further edits before the final version of record.

Subject of Research: How combined exogenous IAA and 6-BA applications regulate sprout development and endogenous hormone balance in stumped Pinus yunnanensis seedlings.

Article Title: Regulatory effects of combined application of exogenous IAA and 6-BA on sprout development and endogenous hormone balance in Pinus yunnanensis seedlings after stumping

Article References: Xu, W., Geng, S., Lu, M., Kang, X., Xu, Y., Cai, N., & Yang, B. (2026). Regulatory effects of combined application of exogenous IAA and 6-BA on sprout development and endogenous hormone balance in Pinus yunnanensis seedlings after stumping. BMC Plant Biology. https://doi.org/10.1186/s12870-026-10248-8

Image Credits: AI Generated

DOI: 10.1186/s12870-026-10248-8

Keywords: Pinus yunnanensis, stumping, sprout development, IAA, 6-BA, endogenous hormones, GA3/ZA ratio, clonal propagation, plant hormones, path analysis, BMC Plant Biology, Regulatory